In technical applications, such as, for example, automobile construction and the like, it can be necessary to quickly and reliably detect the location of a component that can be moved into two end positions relative to a stationary part, using measurement technology. In the case of a belt lock of an automobile, it can be checked, for example, whether a passenger is belted or not. Knowledge of the state of the belt lock is useful to notify a passenger by a signal to put on and fasten the seat belts. Since the introduction of airbags, information about the closed state of the seat belts has also been useful for activating or deactivating mechanisms for inflating driver and passenger airbags or side and head airbags.
For example, in known applications, there are Hall sensors for contactless monitoring of the components that change their location, for example, ones that can assume two different end positions. Hall sensors can include a semiconductor layer supplied with a constant current, in a known integrated construction. A magnetic field component perpendicular to the semiconductor layer can influence the constant current, and the sensor delivers a Hall voltage that can be evaluated, tapped and used to evaluate a status, or that can also be used directly as a switching voltage. The integrated construction of Hall sensors can make it possible to integrate an evaluation circuit that is suitable for evaluation of the switching state on the Hall sensor. In the automotive industry, therefore, Hall sensors can be used as contactless status sensor in many applications.
EP-A-0 861 763 discloses, for example, a belt lock with an integrated, pretensioned Hall sensor that without contact detects the state of a locking body or an ejector for a lock tongue that has been inserted into the belt lock. Here, a Hall sensor with a Hall field can be located in direct proximity to a permanent magnet. Changing the location of the locking body and of the ejector that includes a ferromagnetic material for this purpose changes the magnetic field of the permanent magnet. In doing so, the signal of the Hall sensor changes, and at the output of the Hall sensor, the status change can be tapped as a voltage change. In one embodiment, it is suggested that the Hall sensor with a Hall field be installed without a permanent magnet and the locking body or the ejector be designed as a permanent magnet for this purpose. In this arrangement, the change in the position of the locking body or of the ejector will also be detectable by the change of the Hall voltage.
A disadvantage in the belt lock according to EP-A-0 861 763 is that the Hall sensor must be positioned very carefully with reference to the locking element or the ejector. Subsequent installation of the Hall sensor is therefore relatively complex and expensive. Depending on its arrangement, the Hall sensor is also sensitive to external stray electromagnetic fields that can be caused by, for example, a magnetic key ring. Optionally, even additional shielding must be mounted. This further complicates the structure or the installation. The susceptibility to external stray fields can also be increased by the signal changes being relatively small due to the relatively short paths that must be traversed by the locking body or the ejector when the seat belt lock is locked or unlocked. The seat belt variant without a pretensioned Hall sensor in which either the locking body or the ejector is designed as a permanent magnet is less practicable. The attainable signal changes are also relatively small here. This can make it difficult to detect different states, for example, the belt lock locked or unlocked. Vibrations of the locking body and of the ejector during locking and unlocking of the seat belt can cause demagnetization of the permanent magnet with time. This can ultimately lead to the Hall sensor becoming ineffective and the status changes of the belt lock no longer being able to be reliably detected.
The known belt locks can have a compact design that therefore can seriously limit the available space within the belt lock. This makes it difficult to arrange sensor components within the belt lock housing, especially in the vicinity of a component that changes its location from one end position into the other end position when the belt lock is actuated. If shields are also to be attached, the engineer is generally faced with an essentially insoluble problem because the dimensions of the belt lock housing are not to be changed.